1. Field of the Invention
The present invention relates to a bootstrap pilot circuit in N-MOS technology for capacitive loads.
2. The Prior Art
A bootstrap circuit is known to be a circuit capable of bringing about an oversupply of the gate of the load transistor for a high conduction of said transistor.
Due to the effect of these characteristics the use of bootstrap circuits is frequent in N-MOS technology when it is desired to pilot with short response time a capacitive load of considerable value. The high conduction of the load transistor developed with the bootstrap circuit makes it possible to have rapid commutation of the output from low to high level.
A typical bootstrap circuit calls for separate piloting (but made appropriately sequential) of a load transistor and of a drive transistor in series and the interposition of a bootstrap condenser between the gate of the load transistor and the output, coinciding with an intermediate point between the two transistors.
With the drive transistor on and the load transistor off a pilot signal is first sent to the load transistor gate, which goes on, bringing about a simultaneous conduction condition of the two transistors. The same signal brings about loading of the bootstrap condenser, which holds the load transistor gate high. A second pilot signal is then sent to the drive transistor gate, which turns off, allowing the load transistor to make the output level rise and thus bring the load transistor gate higher due to its superconduction.
This known circuit has several drawbacks which can be summarized as (a) strong absorption of current between one pilot signal and the next due to the simultaneous conduction effect of the two transistors, (b) the need to oversize the drive transistor to obtain in the abovesaid phase as well as a good output voltage at low level, and (c) sequential piloting of two inputs with the resulting signal timing problems, in general by the use of delay lines which can be rather complex and cumbersome.